Dortmund defender Manuel Akanji stressed the importance of Reus to the German club’s cause.
“It would be a big loss, he is our captain and played very well recently, scoring two goals,” Akanji said Tuesday.
“He knows what it takes to make it through to the next round.”
Terzic said Dortmund need “a mix of courage and patience” against City, who are bidding to reach the semi-finals after exiting in the quarters in each of the last four seasons.
Rumours persist that Dortmund could lose 20-year-old striker Erling Braut Haaland to a big-money offer this summer.
Dortmund have made it clear they do not want to sell Haaland, who reportedly has a release clause in his contract for 75 million euros ($89 million) from 2022.